Gas absorption experiments across the gas-liquid interface in a capillary tube were carried out to investigate the mass transfer efficiency in a micro channel. The test channel used was a glass capillary tube having a diameter of 0.5 mm and a length of 30 ram or 200 mm. Carbon dioxide (CO2) and distilled water were supplied into the capillary tube. The now rates were varied widely to form various types of flow patterns. The CO2 absorption rate was measured by a total organic carbon meter. The results show that the gas absorption rate increases remarkably with the superficial gas velocity. The volumetric mass transfer coefficient, k(L)a, and the superficial gas velocity generally have a linear relationship, regardless of the superficial liquid velocity. The change of flow pattern does not drastically affect the k(L)a profile. The values of k(L)a in a micro-tube are generally larger than those measured by conventional gas absorption equipment.